Electrocatalysts have been developed for various applications, such as hydrogen generation based on water splitting. Recently, oxides such as perovskites and other inorganic compounds have been studied for this application. Among the advantages of using perovskites and other oxides compared to noble metals such as platinum is reduced cost and structural flexibility. In an ABO3-type (perovskite) structure, both A- and B-sites can be varied by using metal cations with different valences or ionic radii. Partial replacement or doping at these sites is relatively straightforward and may result in a change of chemical, electronic, and physical properties. During the past several years, a series of perovskites have been studied as electrocatalysts for oxygen evolution reaction (OER), which is utilized in water oxidation electrodes in solar-fuel conversion devices to generate hydrogen fuel from water. Typically, the reported OER catalysts are used in alkaline media. In acidic media, the OER may be extremely slow due to a sluggish reaction process, which poses a technological hurdle for producing hydrogen fuel from water. In addition, the currently known OER catalysts also suffer from a lack of stability in acid. However, the acidic condition is important for full solar-fuel devices because a faster, more efficient hydrogen evolution reaction (HER), which occurs at the other half electrochemical cell, can be achieved only in an acid environment.
To date the most successful OER electrocatalysts under acidic conditions are ruthenium oxide (RuO2) and iridium oxide (IrO2), despite of years of research in this area. Although it is attractive due to its high OER catalyst activity, the RuO2 catalyst is oxidized to ruthenium tetroxide (RuO4) in acidic media during use (>1.4V) and RuO4 is readily dissolved into the electrolyte. Thus, the catalyst loses its electrocatalytic activity substantially during the water splitting reaction. On the other hand, while it is highly corrosion resistant (stable at 1.95 V vs. RHE for >240 h in 0.5 M H2SO4), the IrO2 electrocatalyst has a much lower OER activity than does RuO2. Thus, industrial application of solar-water splitting devices continues to be hindered by the lack of stable electrocatalysts having an OER activity similar to or better than that of RuO2.